This invention relates to a throttling control choke valve. More particularly, this disclosure pertains to a wider operating range choke valve and reduced valve seat and stem seat erosion. The disclosure also pertains to valve body erosion and corrosion resistance and detection.
Variable choke valves or fluid flow regulation valves are used in flow lines leading from oil and/or gas producing wells and sometimes in flow lines leading to water injection wells. These flow line chokes have movable means for varying the amount of restriction to be imposed by the choke on the fluids flowing through the flow line. Three types of movable means are in general use. One type valve uses dual parallel discs with a circular aperature in each disc. The maximum flow rate is dependent on the size of the hole and throttling action is obtained by varying the amount of alignment of the holes. A second type of choke valve uses a cylindrical perforated cage in combination with a cylindrical piston or a cylindrical sleeve. Either the piston or sleeve or the cage is movable to vary the size and number of perforations through which the fluids may flow from the inlet to outlet side of the valve. The third type of valve employs a valve stem with a tapered end which is moved in and out of a similarly tapered member in the valve to vary the amount of opening through which the fluids may flow. This type of choke valve is commonly referred to as a needle and seat valve. Without a change of inner members, these common choke valves have limited effective flow rate range abilities, generally operating between 30% and 80% of full capacity. Ideally a choke control valve should be able to control the flow of liquids and gases, including entrained particles, over a range from a few percent to 90% at variable pressure differentials without the need for changing inner valve choke members. For example, some oil and gas producing operations occur in very cold areas where it is important to maintain sufficient liquid flow, for example, about 5%, to prevent freezing or congealing of the liquid in the line.
The requirements for choke valves are among the most severe, for any valve service choke control valves are subject to greatly aggrevated erosion and/or corrosion of the inner surface of the valve body and the inner valve members in the valve just before, at, and just downstream of the point of throttling. The aggrevated erosion and corrosion is a combination of several conditions. Petroleum fluids frequently contain sand-like mineral particles, brine and acid gases. The erosive and corrosive characteristics of petroleum fluids in throttle valves is aggrevated by the effects of increased fluid turbulence, impingement on metal surfaces and fluid cavitational phenomena. Ideally the valves should have long life and should be easy to maintain. Choke valve designers have tried without total success to eliminate erosion and corrosion. It has become common practice to use erosion and corrosion resistant pistons, sleeves, cages, tapered stem tips, and valve seats made of or plated or lined with tungsten carbide, chrome stainless, stellite and ceramics. It has also become common practice to line the valve body inlet, chamber and outlet with such erosion and corrosion resistant materials. Despite these preventive measures, valve seats and members still continue to erode altering the control characteristics and abilities of the valve and frequently rendering the valve unreliable. In addition, valve bodies still continue to fail. Failures are dangerous, environmentally undesirable and clean ups and repairs costly. Routine X-ray, ultrasound and other procedures for detecting body erosion, pitting or other forms of metal loss are rendered unrealible by the such carbide and other preventive materials. Ideally the choke valve should be designed in a way that minimizes erosion of the critical valve shut off points and of surfaces on the movable part of the valve and on mating seat surfaces. Moreover, ideally metal loss adjacent the inner surfaces of the valve should be detectable before substantial erosion and corrosion of the valve body occurs.
Accordingly, it is the primary object of this invention to provide a variable choke control and shutoff valve leaving the aforementioned ideal charcteristics.